The formation of flexographic printing plates by the process of pressing the flexographic plate material into a matrix material in a press is well known. A requirement now exists for relatively rigid plastic letterpress printing plates, and the formation of these plastic letterpress printing plates present several problems not encountered in the formation of flexographic plates. One particular problem area is in the matrix material used to form the plastic letterpress printing plate.
The procedure for the manufacture of plastic letterpress printing plates from plastic resins involves forming a matrix, and then molding the plastic plate material to the matrix. The matrix is made from a fiber reinforced thermosetting resin material known in the art as matrix board. The matrix board is molded to a patterned master plate and cured with heat and pressure to form a mold for the plastic letterpress plates, which are typically formed from thermoplastic material. The molded matrix will have relief portions, i.e. impressions, and flat or dead areas. The relief portions correspond to the raised printing areas which will be obtained on the plastic letterpress printing plates formed from matrix, and the flat or dead areas will correspond to the non-printing areas on the plastic letterpress plates. The relief and flat portions of the matrix will sometimes be referred to herein as "printing" and "non-printing" areas, respectively. The molded matrix is inserted into the metal cavity of an injection molding press, and the thermoplastic resin, such as polypropylene, is injected into the matrix mold at high temperature and pressure. For example, in forming plastic letterpress printing plates from polypropylene thermoplastic material, temperatures in the range of 400.degree. F and pressures in the range of 6000 psi may be encountered.
Some significant problems are encountered in the molding of such thermoplastic letterpress printing plates. The molding pressure is applied by a press ram which injects the thermoplastic molding material against the matrix. One significant problem is that the rate of heat transfer through the matrix is slower than through the metal (usually steel) surface of the injection molding press mold. Thus, the overall cooling rate, and hence the cycle time for plate formation, is a function of the thickness and thermal conductivity of the matrix. Thicker matrix materials increase the cycle time and thus increase the cost of plate making. In addition, the different heat transfer rates through the matrix material and the press plate result in uneven cooling of the opposed surfaces of the plastic letterpress printing plate, and this uneven cooling leads to undesirable warpage of the plastic letterpress plate when the plate is removed from the press.
Attempts have been made to achieve a thin molded matrix simply by reducing the thickness of the starting matrix material, i.e. the unmolded matrix board. However, when matrix thickness is reduced to facilitate heat transfer, there is a significant reduction in densification of the non-printing areas of the matrix board during matrix formation. These low density non-printing areas tend to compress at the pressure levels used in subsequent plate molding, with the result that there is a loss of relief between the printing and non-printing areas. This loss of relief or contrast between the printing and non-printing areas increases the tendency of non-printing areas to pick up ink and to print where printing is not desired.